Sail apparatus and extendible member suitable for use in the sail apparatus

ABSTRACT

There is discussed a sail apparatus having a sail with one or more battens. A furling system varies the area of deployed sail between a furled state and an unfurled state by rotating the sail about the leading edge. At least one batten comprises a single extendible sheet member operable to switch between a rigid three-dimensional longitudinal state and a coiled planar state, such that a deployed portion of said at least one batten is in said rigid, three-dimensional longitudinal state and a furled portion of said at least one batten is in a coiled planar state. The batten is resiliently baised towards the leading edge of the sail, while allowing movement of the batten towards the trailing edge of the sail during furling.

PRIORITY APPLICATIONS

This application claims priority from UK patent application no. 1119750.6 filed on 15 Nov. 2011 and from UK patent application no. 1200248.1 filed on 6 Jan. 2012, which are both incorporated herein in their entirety by reference.

BACKGROUND OF THE INVENTION

b 1. Field of Invention

In one aspect, this invention relates to a sail apparatus, and in particular to a sail apparatus in which a sail is furled and unfurled about its leading edge. In another aspect, this invention relates to an extendible member which may be used as a batten in the sail apparatus, and a method of manufacture therefor.

2. Description of Related technology

It is well known that battens can increase the area of a sail beyond the area which is directly supported by the tension within the sail. The additional area, sometimes referred to as the roach of the sail, is supported by the rigidity of the battens to maintain the desired aerodynamic profile. Battens are typically located toward the trailing edge of the sail and their main axis intersects the leading edge, sometimes referred to as the luff edge, of the sail.

A problem occurs when it is desired to use battens in combination with a sail rig in which the sail is furled/unfurled by winding/unwinding the sail around an axis which runs along the leading edge of the sail. Such a ‘roller-furling’ sail rig allows the area of deployed sail to be easily and quickly varied. However, the rigidity of conventional battens is incompatible with winding the sail about the leading edge.

At least two different techniques have been used to address this problem. U.S. Pat. No. 4,633,798 discusses using battens formed by a thin, resilient material having a curved, stable state but being deformable into a planar state for furling. In practical implementations, it has been typically necessary to employ two such battens facing each other, but even with two battens the stiffness is not particularly high. In addition, the stresses applied to the battens in the curled state as a result of the resilience of the battens tends to weaken the battens. Further, the battens may damage the sail material.

WO 94/14648 discusses an alternative technique utilising inflatable battens. The need for inflation/deflation equipment makes this technique expensive, and also prone to mechanical failure. Further, the time taken to inflate and deflate the battens make them unsuitable for competitive sailing, and in certain circumstances could be a safety concern.

SUMMARY OF THE INVENTION

An object of the present invention is to provide an alternative solution to the problem of using battens with a ‘roller-furling’ sail rig. Aspects of the invention are set out in the accompanying claims.

An embodiment of the invention comprises using sail battens formed by a single extendible sheet member which has two stable states: a rigid three-dimensional longitudinal state in which the cross-section of the sheet member generally forms an arc; and a coiled planar state in which the cross-section of the sheet member is generally linear. In use, the extendible sheet member switches between the rigid three-dimensional longitudinal state and the coiled planar state at a position which varies in response to winding/unwinding the sail about its leading edge. In the rigid, three-dimensional longitudinal state, the extendible sheet member stiffens the sail to provide an effective roach area. The coiled planar state allows the sail to be furled/unfurled using a ‘roller-furling’ sail rig.

WO 97/35706, the whole contents of which are hereby incorporated by reference, discusses an extendible sheet member which can be adapted for use in the present invention. The extendible sheet member of WO 97/35706 has two stable states, a tubular extended state and a planar coiled state. Transition from the tubular extended state to the planar coiled state is effected by manually manipulating one end of the sheet member to have a planar cross-section. For the purposes of the present invention, such direct manual manipulation is not feasible. Accordingly, in a preferred embodiment, the extendible sheet member of WO 97/35706 is modified by chamfering one end when in the tubular state. In this way, the act of winding/unwinding the sail about the leading edge causes the transition point between the tubular extended state and the coiled planar to move along the length of the extendible sheet member so as to remain adjacent to the leading edge as the sail is furled/unfurled without the need for any direct manual manipulation of the extendible sheet member. If the cross-section of the sheet member in the tubular state extends more than 180°, then the chamfered end may be splayed to ensure that at a transition point between the rigid tubular extended state and the coiled planar state, the cross-section of the sheet member forms an arc having an extent of not more than 180°.

The modifications made to the extendible sheet member of WO 97/35706 are considered to be inventive independent of the sailing application, and would be advantageous in other applications where direct manual manipulation of the extendible sheet member is either unfeasible or undesirable.

In another embodiment, there is provided a sail having a roller-furling rig, with sail battens mounted to the sail. The sail battens are mounted to allow the battens to move aft as the sail furls. Preferably, this is achieved using a resilient mounting.

BRIEF DESCRIPTION OF THE DRAWINGS

By way of example, various embodiments of the invention will now be described with reference to the accompanying figures, in which:

FIG. 1 schematically shows sail apparatus according to the present invention attached to a mast;

FIG. 2 schematically shows a sectional view through the sail apparatus and mast of FIG. 1 with the sail in an unfurled state;

FIG. 3 schematically shows a sectional view through the sail apparatus and mast of FIG. 1 with the sail in a partially furled state;

FIG. 4 schematically shows a batten member forming part of the sail apparatus of FIG. 1, with one end of the batten member in a planar coiled state and the other end of the batten member in a three-dimensional longitudinal state;

FIGS. 5A and 5B schematically show views of a batten member forming part of the sail apparatus illustrated in FIG. 1, with the entire length of the batten member in the three-dimensional longitudinal state;

FIG. 6 schematically shows the batten member illustrated in FIG. 4 partially wrapped around a foil;

FIGS. 7 and 8 show schematic views of an inner end of the batten member illustrated in FIGS. 5A and 5B in situ in a sail pocket in an unfurled state;

FIG. 9 shows a section through the lines A-A illustrated in FIG. 8;

FIGS. 10 and 11 show schematic views of the inner end of the batten member illustrated in FIGS. 5A and 5B in situ in a sail pocket in a partially furled state;

FIGS. 12A and 12B respectively show schematic sectional views through the lines B-B and C-C illustrated in FIG. 10;

FIG. 13 schematically shows sail apparatus according to the invention attached to a stay;

FIG. 14 schematically shows a sectional view through the sail apparatus and stay of FIG. 13 in an unfurled state;

FIG. 15 schematically shows a sectional view through the sail apparatus and stay of FIG. 13 in a partially furled state;

FIG. 16 schematically shows a cross-sectional view of through the batten member;

FIGS. 17A and 17B show plan views of an alternative batten member;

FIGS. 18A to 18C respectively show cross-sectional views through different positions along the alternative batten member shown in FIGS. 17A and 17B;

FIGS. 19A and 19B schematically show the trailing end of the alternative batten member affixed to the sail;

FIGS. 20 and 21 respectively show side and plan views of another alternative batten member;

FIGS. 22A to 22C show cross-sections through the alternative batten member of FIGS. 20 and 21 at three different longitudinal positions;

FIG. 23 shows the alternative batten member of FIGS. 20 and 21 mounted to a sail;

FIG. 24 shows the alternative batten member of FIGS. 20 and 21 mounted to a sail with the sail pocket removed for illustrative purposes;

FIG. 25 shows in more detail the mounting of the leading edge of the batten member of FIGS. 20 and 21 to a sail;

FIGS. 26 and 27 show in more detail the mounting of the trailing edge of the batten member of FIGS. 20 and 21 to a sail;

FIGS. 28A and 28B respectively show a plan view and a side cross-sectional view of the batten member mounted at a first relative position to the sail;

FIGS. 29A and 29B respectively show a plan view and a side cross-sectional view of the batten member mounted at a second relative position to the sail;

FIG. 30 shows an isometric view of a further alternative batten member;

FIG. 31A shows a side view of the further alternative batten member illustrated in

FIG. 30, and FIGS. 31B and 31C respectively show schematic sectional views through the lines K-K and L-L illustrated in FIG. 31A; and

FIGS. 32 and 33 respectively show isometric and plan views of the batten member of FIGS. 30 and 31A to 31C attached to a sail.

DETAILED DESCRIPTION OF THE INVENTION

A first embodiment of the invention will now be described with reference to FIGS. 1 to 12. As shown in FIG. 1, in the first embodiment a sail 1 is a mainsail mounted to a mast 3 and a boom 5. The sail 1 is provided with a plurality of battens 7 a-7 d to maintain a roach area of the sail in an aerodynamic profile where the tension within the sail would not normally do so. A ‘roller-furling’ sail rigging is employed in which, as shown in FIG. 2, the leading edge 9 of the sail 1 is threaded onto a rotatable foil 15, and the sail 1 is wound and unwound about its leading edge 9 so that the furled portion of the sail 1 is within the mast 3 (see FIG. 3). The acts of winding and unwinding the sail 1 can be referred to as rolling.

The furling system of this embodiment is provided with actuators (not shown) which apply a force to the trailing edge 11 of the sail 1 in a direction along the boom 5 away from the mast 3 to unfurl the sail 1 and which apply a rotational force to the foil 15 to furl the sail 1. These actuators may either be motorised, pneumatic or manually driven using, for example, a winch.

FIG. 2 shows a cross-section through the foil 15 and the sail 1 when the sail 1 is in an unfurled state. As shown, the batten 7 is held in a pocket 21 formed on or in the sail 1. In the unfurled state, the batten is in a rigid longitudinal state. The sail 1 is furled by rotating the foil 15 about a roll axis, and the pocket 21 is formed on the side of the sail 1 which faces inwards toward the roll axis when the sail 1 is furled. FIG. 3 shows a cross-section through the foil 15 and the sail 1 with the sail in a partially wound state. The pocket 21 is not shown separately from the batten 7 in FIG. 3 for ease of illustration. The portion of the batten 7 which is furled around the foil is in a planar coiled state. A feature of the extendible sheet members used in this embodiment is that they are stable both in the three-dimensional rigid state and the planar coiled state, and can transition many times between those two states without excessive wear. Accordingly, the sail and battens can be furled and unfurled many times before needing to be replaced.

In this embodiment, the battens are made using the extendible sheet members discussed in WO 97/35706. Such extendible sheet members are available from Rolatube Technology, 130 Wellworthy Road, Ampress Park, Lymington, 5041 8JY, UK. In this embodiment, various modifications are made to the extendible sheet member of WO 97/35706 to make the extendible sheet member more suitable for use as a sail batten. FIGS. 4, 5A and 5B show batten members after such modification. As shown in FIG. 4, an end 25 of the batten 7 which, in use, is proximate the trailing edge 11 of the sail 1 has a rounded profile in a plane normal to the surface of the sail 1 and along the main axis of the batten 7. In this way, when the end 25 of the batten 7 is in the planar coiled state, there are no sharp edges which may damage the sail material and the transition from the three-dimensional longitudinal state to the planar coiled state is facilitated.

In order to facilitate furling of the sail 1, in this embodiment the end 27 of the batten 7 which, in use, is proximate to the leading edge 9 of the sail 1 is modified to facilitate initiation of the transition from the three-dimensional longitudinal state to the planar coiled state. In particular, as shown in FIGS. 5A and 5B, the end 27 of the batten 7 is chamfered in the three-dimensional longitudinal state. In other words, the end surface at the end 27 of the batten 7 adjacent the leading edge 9 of the sail 1 is obliquely angled with respect to a cylindrical cross-sectional surface of the batten 7 in the three-dimensional longitudinal state. The exact angle will depend on various factors such as the radius of the foil 15 about which the sail 1 is wound, but typically the length of the chamfered portion is greater than half the width of the batten 7 when in the planar state, and further may be greater than 100 mm. In addition, the end 27 of the batten 7 adjacent the leading edge 9 of the sail 1 is splayed in the three-dimensional longitudinal state so that the arc formed by the cross-section of the extendible sheet member extends over a smaller angle than in the central portion of the batten 7 in the three-dimensional longitudinal state. In particular, the end of the batten 7 adjacent the leading edge 9 of the sail 1 is splayed such that at the transition between the three-dimensional longitudinal state and the planar coiled state the cross-section of the extendible sheet member forms an arc having an extent of not more than 180°. This allows the extendible sheet member to be transitioned between the three-dimensional longitudinal state and the planar coiled state simply by the action of winding the sail about its leading edge as the action of winding tends to cause the extendible sheet member to open up into a planar configuration rather than having the widthwise edges of the extendible sheet member being forced inwards toward the body of the extendible sheet member.

FIG. 16 shows an arc 61 representing the cross-section of the extendible sheet member at the transition point between the three-dimensional longitudinal state and the planar coiled state. As shown, a first line 63 that is tangential to one end of the arc 61 intersects a second line 65 that is tangential to the other end of the arc 61 at a point on the same side of a line drawn between the two ends as the extendible sheet member. Preferably, the acute angle between the first line 63 and the second line 65 is at least 10°.

In this embodiment, the end 27 of the batten 7 proximate the leading edge 9 of the sail 1 is splayed by a thermoforming process in which the end is heated, shaped using a mould and then allowed to cool into the new shape.

For illustrative purposes, FIG. 6 shows how the batten 7 may be partially wound around the foil 15 in the planar coiled state and partially extending rigidly in the three-dimensional longitudinal state.

FIGS. 7 and 8 show the end 27 of the batten 7 proximate the leading edge 9 of the sail 1 in the pocket 21 with the sail 1 in an unfurled state. As shown, the batten 7 has a chamfered portion 35, which is closely accommodated in a first tapered pocket portion 37. FIG. 8 clearly shows that the chamfered portion 35 of the batten 7 has been splayed open. The main portion 39 of the batten 7 is mounted in a second pocket portion 41 which is sufficiently wide to accept the batten 7 in its planar state. In this embodiment, the batten 7 has a diameter of 40 mm in its three-dimensional longitudinal state and the width of the third pocket portion is just over 120 mm. FIG. 9 shows a cross-section through the main portion 39 of the batten 7 and the second pocket portion 41 when the batten 7 is in the three-dimensional longitudinal state. As shown in FIG. 9, the cross-section of the extendible sheet member forms an arc of almost 360° so that the batten 7 is generally tubular.

FIGS. 10 and 11 show the sail in a partially furled state such that the end 27 of the batten has started to wind around the foil 15. As shown in FIG. 12A, the portion of the batten 7 away from the foil still has a generally tubular profile. However, as shown in FIG. 12B, closer to the foil 15 the batten 7 has a more open, planar configuration in which the cross-section of the extendible sheet member forms an arc of less than 180°.

In this embodiment, the pocket 21 is formed of an elastic material. In this way, the pocket 21 contracts against the sail 1 during furling, allowing the furled sail to be more compact. However, using an elastic material for the pocket 21 is not essential.

A second embodiment will now be described in which the trailing end of the batten 7 is modified to facilitate passage through a narrow slot in a mast during an unfurling operation. As shown in FIGS. 17A and 17B, in the second embodiment the trailing end 71 of the batten 7 has a chamfered portion, in a similar manner to the leading end 73, which is terminated by a tab portion 75. The degree of chamfer is set so that if the batten 7 comes into contact with the sides of the slot in the mast during an unfurling operation, the cross-section of the batten 7 at the point of contact is in the form of an arc having an extent of less than 180°. In this way, the sides of the slot do not act to deform the batten 7 by forcing the widthwise edges of the extendible sheet member into the body of the sheet member,

FIGS. 18A, 18B and 18C respectively show cross-sections through the extendible sheet member at positions A-A, B-B and C-C in FIG. 17A. As shown in FIG. 18A, in the tab portion the cross-section is only slightly curved. As shown in FIG. 18B, in the central portion of the extendible sheet member the arc extends close to 180° so that the extendible sheet member is generally tubular, while FIG. 18C shows the cross-section of the extendible sheet member when the cross-section forms an arc of 180°.

As shown in FIG. 17A, an aperture 77 is formed in the centre of the chamfered portion adjacent the trailing end 71. When the batten 7 is mounted to the sail, the leading end 73 is inserted into a tapered pocket portion and then, as shown in FIG. 19A to 19B, the centre of an elastic cord 81 is fed through the aperture 77 and then looped around the tab portion 75. The ends of the elastic cord 81 are fixed to the sail so that the elastic cord applies a resilient force to the batten 7 which acts to press the leading end 73 into the tapered pocket portion. In this embodiment, a conventional batten end protector 83 is then fixed to the sail over the tab end portion so that the tab end portion is captured between the batten end protector and the sail. The batten end protector 83 is positioned to allow some axial movement of the batten 7, but this is limited by the resilient force applied by the elastic cord 81. When the batten 7 is furled, the trailing end of the batten 7 moves against the resilient force applied by the elastic cord 81 further into the recess between the batten end protector and the sail. This rearward movement of the batten 7 as the batten 7 is wrapped around the leading edge of the sail prevents the batten 7 from pulling away from the surface of the sail during furling, due to the differing radii of the furled sail and the furled batten, which is advantageous as otherwise the sail would not wrap compactly about its leading edge.

In the first and second embodiments, in the three-dimensional rigid state the cross-section of the batten over the central portion of its length is almost circular. A third embodiment will now be described with reference to FIGS. 20 to 29 in which in the three-dimensional rigid state, the cross-section of the batten over the central portion of its length forms an arc having a central angle of less than 180°. Such a batten may be made by making a longitudinal cut along the length of an extendible sheet member as used in the first and second embodiments, prior to chamfering and rounding the ends. It will be appreciated that the cross-sectional arc need not be exactly the arc of a circle, and for example could be the arc of an ellipse.

As shown in FIGS. 20, 21 and 22A-22C, the batten 101 of the third embodiment has a chamfered end 103 and a rounded end 105. When mounted to a sail, the chamfered end 103 forms the leading end of the batten 101, and facilitates the transition from the longitudinal three-dimensional state to the planar coiled state as the sail is furled. With the reduction in the extent of the arc formed by the cross-section of the batten 101, splaying of the chamfered end 103 is not performed or required.

When mounted to a sail, the rounded end 105 forms the trailing end of the batten 101. As shown in FIG. 21, two apertures 107 a and 107 b are formed toward the rounded end 105 of the batten 101. The two apertures 107 are located generally centrally relative to the width of the batten 101, but axially separated relative to the longitudinal axis of the batten 101.

As shown in FIGS. 23 and 24, the batten 101 is mounted to a sail 111 in a sail pocket 113 (not shown in FIG. 24), with the rounded end 105 adjacent the trailing edge 115 of the sail 111. As shown more clearly in FIG. 25, the chamfered end 103 of the batten 101 slidably engages a pocket member 117 which is attached to a reinforced region 119 of the sail 111.

As shown more clearly in FIGS. 26 and 27, and the rounded end 105 of the batten 101 is attached to the sail 111 by an elastic cord 121 which is threaded through a reinforced region 123 of the sail 111 and through the apertures 107 in the batten 101, and secured via a button member 125. With reference to FIG. 27, two grommets 127 a, 127 b are fixed to the sail (not shown in FIG. 27) and the elastic cord 121 is threaded through the button member 125, one of the apertures 107, one of the grommets 127 and then back through the other grommet 127, the other of the apertures 107 and the button member 125 and secured. Each end of the elastic cord 121 can be secured in various different manners, for example by tying both ends together or by tying knots in each individual end which abut against, but will not pass through, the button member 125.

As shown in FIGS. 28A and 28B, if the batten 101 moves in a direction towards the trailing edge 115 of the sail 111, the elastic cord 121 extends and thereby applies an elastic force to the batten 101 in the direction of the leading edge of the sail 111. In this way, the elastic cord 121 provides a bias force to push the chamfered end 103 of the batten 101 into the recess formed between the pocket member 117 and the sail 111, but allows movement of the batten 101 towards the trailing edge 115 of the sail 111 during a furling operation to allow reliable furling of the sail 111 about its leading edge. FIGS. 29A and 29B illustrate the configuration of the batten 101 and the sail 111 when the batten 101 is pushed fully into the recess formed between the pocket member 117 and the sail 111.

Although the extendible sheet members used to form the battens of the first to third embodiments have sufficient stiffness to support the roach area of the sail in most applications, in some applications it may be desirable to further stiffen the battens. To address this, a fourth embodiment will now be described with reference to FIGS. 30 to 33. In the fourth embodiment, the batten 101 of the third embodiment is stiffened by the addition of a reinforcing member 151. Features of the third embodiment have been referenced by the same reference numerals in FIGS. 30 to 33 and will not be described in detail again.

As shown in FIGS. 30 to 33, the reinforcing member 151 is formed by a strip of material having a length which extends along the majority of the length of the batten 101, but not extending over a length adjacent to the rounded end 105 of the batten corresponding approximately the circumference of the batten 101 to improve the stability of the batten in the planar coiled state. The width of the reinforcing member 151 is slightly less than the widthwise extent of the batten 101 in the longitudinal three-dimensional state. As best shown in FIG. 31A, in this embodiment the reinforcing member 151 is attached to the batten 101 by an elastic cord 153 which is stitched through the reinforcing member 151 and the batten 101 in a coarse running stitch pattern.

In this embodiment, the reinforcing member is formed by a plastic member formed into a two-dimensional arc across its major length. . Other materials may, however, be used, but it is crucial that they are manufactured from a material that has mechanical properties that allow it to be rolled with the batten, many times without degradation.

Modifications and Further Embodiments

In the first embodiment, the sail 1 is a mainsail whose leading edge 9 is attached to a foil 15 within a mast 3. The invention is applicable to other sail apparatus in which a sail is wound and unwound about its leading edge. For example, as shown in FIGS. 13 to 15, in an alternative embodiment the invention is applied to a headsail 51 which is wound about a foil 53 attached to a headstay 55. In another embodiment, the invention is applied to a sail which is wound about a stay.

The first embodiment discusses one way in which the end of a batten proximate the leading edge of the sail can be structured to facilitate the transition of the batten between the three-dimensional longitudinal state and the coiled planar state by a rolling operation without any direct manual access to the batten. Alternative arrangements are possible. For example, the end of the batten proximate the leading edge of the sail could be permanently maintained in the coiled planar state by keeping a portion of the sail permanently wound about its leading edge.

The modifications to the extendible sheet member of WO 97/35706 made in the first embodiment may have application outside of a sail apparatus to facilitate transition of the extendible sheet member from the three-dimensional longitudinal state to the planar coiled state without direct manual manipulation. For example, such a mechanism could be used to deploy and retract an arm member in an unmanned deep sea vessel, or to retract a probe arm in a hazardous area.

While it is preferred that the chamfered end of the batten be splayed, this is not essential.

While the extendible sheet member of WO 97/35706 is convenient for implementation of the present invention, alternative extendible sheet members which transition between two stable states, one in which the sheet member is in a three-dimensional longitudinal state and the other in which the sheet member is in a planar coiled state, could be used.

In the second and third embodiments, a resilient member provides a bias force to urge the battens towards the leading edge of the sail when unfurled, while allowing the battens to move towards the trailing edge of the sail during furling. Such a resiliently-biased arrangement may be used with other types of battens, not just those employing bistable extendible sheet members as exemplified in the disclosure of WO 97/35706.

In the third embodiment, a button member 119 is utilised but the button member 119 is not necessary and may be omitted. 

1. Sail apparatus comprising: a sail having a leading edge and one or more battens; and a furling system operable to vary the area of deployed sail between a furled state and an unfurled state by rotating the sail about the leading edge, wherein at least one batten comprises an extendible sheet member operable to switch between a rigid three-dimensional longitudinal stable state and a coiled planar stable state, such that when deployed said at least one batten is in said rigid, three-dimensional longitudinal stable state and when furled said at least one batten is in said coiled planar stable state.
 2. Sail apparatus according to claim 1, wherein said rigid three-dimensional longitudinal state has a cross-sectional profile in the shape of an arc.
 3. Sail apparatus according to claim 2, wherein said rigid three-dimensional longitudinal state has a generally tubular profile.
 4. Sail apparatus according to claim 2, wherein said rigid three-dimensional longitudinal state has a profile in the form of an arc having a central angle of 180° or less.
 5. Sail apparatus according to claim 2, wherein the extendible sheet member has an outer end toward the trailing edge of the sail and an inner end toward the leading edge of the sail, wherein said inner end is chamfered to facilitate initiation of the transition from the rigid three-dimensional longitudinal state to the coiled planar state upon rotation of the sail about the leading edge.
 6. Sail apparatus according to claim 5, wherein said inner end is splayed such that a cross section through the cross-section of the extendible sheet member at a transition point between the rigid, three-dimensional longitudinal state and the coiled planar state forms an arc having a central angle of 180° or less.
 7. Sail apparatus according to claim 5, wherein the outer end of the extendible sheet member has a rounded profile in a plane normal to the surface of the sail and along the main axis of the batten.
 8. Sail apparatus according to claim 5, wherein the outer end of the extendible sheet member is chamfered.
 9. Sail apparatus according to claim 1, further comprising a resilient biasing arrangement to urge the batten toward the leading edge of the sail.
 10. Sail apparatus according to claim 9, wherein the resilient biasing arrangement comprises an elastic cord.
 11. Sail apparatus according to claim 10, wherein the elastic cord is threaded through the sail and the corresponding batten.
 12. Sail apparatus according to claim 1, wherein said at least one batten further comprises a reinforcing member.
 13. An extendible sheet member operable to switch between a rigid three-dimensional longitudinal stable state and a coiled planar stable state in which the extendible sheet member is coiled about a coil axis, the extendible sheet member having an end which is chamfered to facilitate initiation of a transition from said rigid three-dimensional longitudinal stable state to said coiled planar stable state in response to rotation of the extendible sheet member about the coil axis from said chamfered end.
 14. An extendible sheet member according to claim 13, wherein said chamfered end is splayed such that a cross section through the cross-section of the extendible sheet member at a transition point between the rigid, three-dimensional longitudinal state and the coiled planar state forms an arc having an extent of not more than 180° .
 15. An extendible member according to claim 13, wherein the other end of the extendible sheet member has a rounded edge.
 16. Sail apparatus comprising: a sail having a leading edge; a furling system operable to vary the area of deployed sail between a furled state and an unfurled state by rotating the sail about the leading edge; one or more battens; and for each of the one or more battens, a resilient biasing arrangement providing a biasing force in the direction of the leading edge of the sail and allowing movement away from the leading edge of the sail as the batten rotates around the leading edge of the sail when transiting from the furled state to the unfurled state.
 17. Sail apparatus according to claim 16, wherein the resilient biasing arrangement comprises an elastic cord.
 18. Sail apparatus according to claim 17, wherein the elastic cord is threaded through the sail and the corresponding batten.
 19. Sail apparatus according to claim 16, wherein at least one batten comprises an extendible sheet member operable to switch between a rigid three-dimensional longitudinal stable state and a coiled planar stable state.
 20. Sail apparatus according to claim 19, wherein the extendible sheet member has an outer end toward the trailing edge of the sail and an inner end toward the leading edge of the sail, wherein said inner end is chamfered to facilitate initiation of the transition from the rigid three-dimensional longitudinal state to the coiled planar state upon rotation of the sail about the leading edge. 